This application relates to telecommunication systems, such as data communication systems, and, more specifically, to a method and apparatus for canceling a direct current (DC) signal component from the output of a precoder in a transmitter of such systems at a preselected or predetermined time, e.g., after the scheduled transmission of data. The precoder and associated circuits of the present invention incorporate a modulo device and a DC tracking and cancellation circuit for driving the DC value of the output of the precoder to zero after a preselected or predetermined time, e.g., after a scheduled transmission of data. The precoder thereby provides a means for reducing or eliminating the adverse effects that a residual DC value at the precoder output at the end of a scheduled transmission has upon subsequent data transmission in the system.
It is well known by those skilled in the telecommunication art that the performance of a data communication system can be significantly improved by using Tomlinson precoding (xe2x80x9cprecedingxe2x80x9d). Typically, precoding eliminates much of the distortion caused by the transmission channel coupling a transmitter to a receiver. Although there are alternatives to precoding, such as use of a decision feedback equalizer (DFE), precoding has advantages in certain data communication systems, particularly when precoding is used in conjunction with coding schemes such as trellis coded modulation (TCM), since the absence of error propagation will result in a more reliable operation of realizable soft-decision decoders.
In conventional data communication systems, an apparatus for precoding (xe2x80x9cprecoderxe2x80x9d) is typically provided and initialized during a training mode when a training signal is sent from a transmitter to a receiver having a DFE. During the training mode, DFE coefficients are generated in the receiver and then transferred back to the associated transmitter via a conventional return channel or back channel. The transferred DFE coefficients are then placed in the precoder""s feedback filter. The transferred coefficients are typically referred to as precoder coefficients or precoder taps.
The precoder in a data communications system receives a data coded signal, e.g., an input data symbol or digital symbol sequence, and converts the data coded signal into the precoder""s output, which comprises a precoded data coded signal or output data symbol or digital symbol sequence. The input data symbol sequence can be generated by use of a variety of conventional modulation methods, such as pulse amplitude modulation (PAM), quadrature amplitude modulation (QAM), carrierless AM/PM modulation (CAP), discrete multi-tone (DMT) and other methods known to those skilled in the telecommunications art. The precoded output signal conventionally passes through a digital-to-analog converter, a shaping filter, and other transmitter circuits, thereby obtaining a transmitter output signal for transmission on the channel.
It is desirable in some data communications systems for the output of the transmitter to have a low DC component, preferably zero, at a predetermined time after a scheduled transmission of a data coded signal. One type of data communication system requiring the output of the transmitter to have a DC value of zero after such predetermined time is a time division duplex (TDD) data communication system. This requirement has heretofore precluded the use of conventional precoding in TDD systems, because a conventional precoder, and therefore the transmitter output, typically includes a non-zero DC value after such predetermined time. Hence, there is a need for an apparatus and method for completely canceling the DC component at the output of a precoder after such predetermined time.
Further, it is desirable for such an apparatus to be easily implemented within the existing structure of a transmitter so as to require no changes in the operation of the associated receiver.
The principal object of the present invention is to provide a means for driving the DC output component of a precoder to zero at a predetermined time, such as, after the end of a scheduled transmission of a data coded signal. By driving the precoder""s DC output component to zero, the DC output component of the transmitter""s line drivers is also driven to zero. Hence, in accordance with the present invention, a TDD data communication system requirement that a precoding transmitter have a DC output of zero, after a scheduled data transmission, is satisfied by driving the precoder""s DC output value to zero at that time.
Another object of the present invention is to provide a means for driving the precoder""s DC output component to zero with little change in the precoder structure, thereby minimizing the need for additional hardware and software.
A further object of the present invention is to provide an apparatus and method that drives the precoder""s DC output component to zero without producing a large transient.
Yet another object of the invention is to provide a DC canceller or compensator for a precoder, which during a scheduled transmission of data continually provides a DC compensation signal for driving the precoder""s DC output component to zero.
An apparatus and method described in the preferred embodiment meets the above and other objects.
In accordance with the present invention, a method is provided for driving the DC output of a precoder in a data communication system to zero at a predetermined time after a scheduled transmission of data. The method includes the steps of: generating a data symbol sequence d(n); subtracting the output f(n) of a precoder filter from the data symbol sequence d(n) to form an adjusted data symbol a(n); adding a level shifting signal r(n) to the adjusted data symbol sequence a(n) to form a level shifted (modulo) data symbol sequence b(n); adding a DC compensation signal q(n) to the data symbol b(n) to obtain an adjusted modulo signal m(n); determining a running sum s(n) of the adjusted modulo signal m(n); setting the value of an output signal p(n) to the value of the adjusted modulo signal m(n) until the predetermined time; and setting the value of the output signal p(n) to the negative value of the running sum s(nxe2x88x921) at the predetermined time.
In another aspect of the present invention, a precoder for use in a transmitter of a data communication system is provided having a modulo device for receiving an adjusted data symbol sequence that is based on an input data symbol sequence and for generating a modulo signal, and a circuit coupled to the modulo device for driving the DC output of the precoder to zero at a predetermined time. The circuit coupled to the modulo device includes: a compensator summer for adding the modulo signal and a compensation signal thereby obtaining an adjusted modulo signal; a compensator means for generating the compensation signal, the compensator means incorporating an integrator for obtaining a running sum of the adjusted modulo signal; and an insertion switch programmed to selectively set the output of the precoder to be the adjusted modulo signal prior to a predetermined time and, at the predetermined time, to set the output of the precoder to be the negative value of the running sum.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.